HAL Id: jpa-00219096https://hal.archives-ouvertes.fr/jpa-00219096

Submitted on 1 Jan 1979

HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, estdestinée au dépôt et à la diffusion de documentsscientifiques de niveau recherche, publiés ou non,émanant des établissements d’enseignement et derecherche français ou étrangers, des laboratoirespublics ou privés.

EFFECT OF POLARITY ON THE IMPULSEBREAKDOWN OF SF6

E. Antonios Vlastós

To cite this version:E. Antonios Vlastós. EFFECT OF POLARITY ON THE IMPULSE BREAKDOWN OF SF6. Jour-nal de Physique Colloques, 1979, 40 (C7), pp.C7-253-C7-254. �10.1051/jphyscol:19797124�. �jpa-00219096�

JOURNAL DE PHYSIQUE CoZZoque C7, suppt6ment au n07, Tome 40, k ~ ~ i l Z e 6 1979, page ~ 7 - 253

EFFECT OF POLARITY ON THE I M W S E BREAKDOWN OF

E. Antonios ~last6s.

l'he Royal Ins t i tu te of TeehnoZogy, S-100 44 S t o ~

Abstract: The ef fec t of polar i ty on the impulse breakdown of SF6 and the influence which a t h i n d i e l ec t r i c coating has on the impulse breakdown f o r both po la r i t i e s wbs studied in the pressure range of 1-7 bara i n a coaxial cylinder electrode confi- guration.

In t roduc t ion : The thought o f es tab l i sh ing a s im i - l a r i t y law f o r the breakdown o f gases dates back t o the end o f the previous century. Paschen i n t h i s thes is , which appeared 1598, was the f i r s t t o estab- l i s h a r e l a t i o n between t h e breakdown vol tage o f a gas and the product o f pressure times the e lec t rode separat ion f o r p lane p a r a l l e l electrodes. The sc ien- t i f i c scept ic ism about t h i s r e l a t i o n , the so c a l l e d Paschen's law, s t a r t e d t o grow when experimental r e s u l t s from measurements w i t h d i f f e r e n t e lec t rode mater ia ls , e lec t rode spacin s and gases become ava i lab le . Soon i t became c?ear t h a t t h e breakdown vol tage of a gas, as determined experimental ly, i s n o t a monotonous s i n v l e valued f u n c t i o n o f t h e pro- duct pressure p times the e lec t rode separat ion d b u t shows a minimum. Fur ther more i t was found t h a t t h e r e l a t i o n between breakdown vol tage and the pro- duct pd, a t values o f pd, which a r e e i t h e r lower o r h igher than t h a t corresponding t o Paschen's mi- nimum, deviates from the l i n e a r one and t h a t the dev ia t ion appears e a r l i e r f o r small e l ectrode sepa- r a t i o n s d. A l l these w e l l known f a c t s are summarized here t o emphasize t h a t from t le beginning i t was a l - ready es tab l i shed t h a t the outcome from t r i a l s made t o determine the breakdown vol tage o f a gas does n o t on ly depend on the composition and i m p u r i t i e s o f the gas b u t a l s o depends on t h e electrodes, on t h e i r c o n s t i t u t i o n , shape and sur face condi t ions (such as roughness and oxide layers ) . The main reason f o r the i n f l u e n c e o f the e lect rodes on the breakdown vol tage was accepted t o be t h e e l e c t r o n emission from the e lec t rode sur face which appears a t e levated macro- scopic e l e c t r i c a l f i e l d s .

I n s l i g h t l y asymmetrical e l e c t r i c f i e l d s , such as the f i e 1 d produced between two coaxia l c y l inders, the re i s experimental evidence t h a t n o t o n l y the e lec t rode shape b u t a l s o t h e e lec t rode sur face con- d i t i o n s are c r u c i a l f o r the outcome o f the measure- ments e s p e c i a l l y when the p o l a r i t y o f the e lec t rode w i t h the smal ler rad ius i s negative. Moreover, f o r e lec t ronega t i ve gases, where t h e breakdown vo l tage i s h igher than t h a t f o r non-electron-attaching ga- ses the dev iat ions from the s i m i l a r i t y o r Paschen's law were found t o be l a r g e r and t o appear a t pres- sures which a re lower than those corresponding t o non-e lect ron-at taching gases. The appl i c a t i o n o f pressur ized e lec t ronega t i ve gases f o r the insu la - t i o n of h igh vo l tage swi tchgear equipment necessi- t a t e d an i n t e n s i v e research on t h e breakdown vol tage o f e lec t ronega t i ve' gases and especi a1 l y o f SF6 which besides i t s favorable d ie1 e c t r i c p roper t ies a1 so processes good arc quenching p roper t ies . CIGRE's study committee 15 on i n s u l a t i n g mate r ia l s has f rcm a l r ~ a d y publ ished work prepared a composite

Paschen's curve f o r SF6 and the dev iat ions from the Paschen's curve a t d i f f e r e n t e lec t rode spa- c ings f o r e s s e n t i a l l y un i form f i e l d s [I I . The de- v i a t i o n s from the Paschen's curve appear a t a mean e l e c t r i c a l f i e l d o f about 20 kV/mm.

Results: I n o rder t o study the e f f e c t o f p o l a r i t y impulse breakdown o f SF6 and the i n f l u e n c e which a t h i n d i e l e c t r i c coa t ing has on the impulse breakdown f o r bo th p o l a r i t i e s extensive measure- ments were performed w i t h coaxia l c y l i n d r i c a l e lect rodes w i t h diameters o f 70 and 30 mm respec- t i v e l y and a leng th over which the f i e l d was un- d i s t o r t e d of 250 mm. F igure 1 shows the 1 % pos i - t i v e l i g h t n i n g impulse (1.2/50 p) breakdown vo l - tage o f SF6 w i t h bare pol ished pure aluminium and w i t h po l i shed and l a t e r anodized pure aluminium i n n e r e lec t rode as a f u n c t i o n o f the gas pressure. For comparison the 1 % breakdown v o l t a l e s o f a i r w i t h anodized e lect rodes are a l s o given. F iqure 2 shows the 1 % neoat ive 1 i ~ b t n i n n i r w l s e !1.2/50 us) breakdown vol tage o f SF6 and a i r w i t h the same type o f e lect rodes as those i n f i g u r e 1. The 1 % break- down probabi 1 i t i e s o f bare and anodized e lect rodes were der i ved by apply ing a hundred impulses on a new e lec t rode p a i r a t each vo l tage l e v e l and by i n - creas ing the vo l tage l e v e l s by small steps u n t i l the f i r s t breakdowns occured. This measuring pro- cedure seemed t o be the most s u i t a b l e s ince i t re - duces the in f luence o f previous breakdowns on the outcome o f r e s u l t s . The p r o f i l e curve o f t h e po- l ished e lect rodes showed t h a t t h e d i f f e r e n c e be t - ween the c r e s t and bottom l i n e s o f the p r o f i l e curve was about 2-4 m. The ox ide l a y e r thickness o f t h e anodized aluminium elect rodes was about 10 m and the d i f fe rence between the c r e s t and bottom l i n e s o f the p r o f i l e curve o f the anodized e lect rodes was about 1 m. The r e s u l t s presented i n f i g u r e s 1 and 2 are b r i e f - l y summarized as fo l lows : (a) The 1 % impulse breakdown vol tage a t 1 bara i s w i t h i n the accuracy o f the experiments almost equal both f o r bare and covered e lect rodes and fo r the two p o l a r i t i e s . (b) A t h igher pressure t h e d i f fe rence between the 1 % impulse breakdown vol tage f o r p o s i t i v e and ne- ga t i ve impulses increases. (c ) The improvement due t o the anodizat ion o f the e lect rodes i s l a r g e r w i t h negat ive p o l a r i t y making t h e 1 % negat ive im- pulse breakdown vol tage almost equal t o t h a t o f bare e lect rodes w i t h p o s i t i v e p o l a r i t y .

A poss ib le explanat ion o f these r e s u l t s i s t h a t the g rea t dev iat ions between p o s i t i v e and negat ive im- pulse breakdown voltages i n gases such as SF and the dev iat ions from Paschen's law i s i n one Rand due t o the e l e c t r o n emission from the sur face and especi a1 l y t o the e l e c t r o n v e l o c i t y d i s t r i b u t i o n and on the o ther hand t o the s t rong reduc t ion o f the e l e c t r o n attachement cross sec t ion of SF6 a t r e l a t i v e l y low e lec t ron energies 121. By p o l i s h i n g and by anodizing the e lect rodes the e l e c t r o n energy

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797124

Aluminium electrodes Aluminium electrodes

Fig. 1 . Dependence on gas pressure of the1 % Fig. 2. Dependence on gas pressure of t h e 1 % p o s i t i v e ' l igh tn ing impulse ( 1.2/50 u s ) negative l igh tn ing impulse (1.2/50 us) breakdown vol tage o f SF6 and a i r with breakdown vol tage of SF6 and a i r with bare pol ished pure aluminium and f o r ano- bare pol ished pure aluminium and f o r ano- dized p l i shed pure aluminium inner elec- dized polished pure aluminium inner elec- t rode. t r o d e .

dis'tribution may change and the energy of the electrons in the t a i l of the distr ibution may be reduced so that the electrons become attached t o the gas molecules and the breakdown voltage 'in- creases.

Summarizing one can conclude tha t the predic- tions of the similari ty law f o r the breakdown vol- tage of gases such as SF6 and the experimental re- s u l t s are not ant i the t ica l but tha t the basic contradiction between the predictions and the experimental results i s mainly due to electrode ef fec ts and to the influence of the measuring procedure on the determination of the breakdown vol tage.

Acknowledgments: The author i s indebted to the Swedish Board fo r Technical Develo~ment fo r f i - nancial support , t o Prof. S. ~usck ' fo r his f ru i t fu l1 inspiration and t o Dr. A. Mikhail fo r the performance of the measurements.

References :

1.. Rein, A., Electra, 32, pp. 44-60 (1974). 2. James, D.R. , Christophorou, L.G. , Proc. of

Int . Symp. on Gaseous Dielectrics. Tennessee, pp. 224-251 (1978).